Heptanuclear antiferromagnetic Fe(III)-D-(-)-quinato assemblies with an S = 3/2 ground state-pH-specific synthetic chemistry, spectroscopic, structural, and magnetic susceptibility studies.

Iron is an essential metal ion with numerous roles in biological systems and advanced abiotic materials. D-(-)-quinic acid is a cellular metal ion chelator, capable of promoting reactions with metal M(II,III) ions under pH-specific conditions. In an effort to comprehend the chemical reactivity of well-defined forms of Fe(III)/Fe(II) toward α-hydroxycarboxylic acids, pH-specific reactions of: (a) [Fe3O(CH3COO)6(H2O)3]·(NO3)·4H2O with D-(-)-quinic acid in a molar ratio 1:3 at pH 2.5 and (b) Mohr's salt with D-(-)-quinic acid in a molar ratio 1:3 at pH 7.5, respectively, led to the isolation of the first two heptanuclear Fe(III)-quinato complexes, [Fe7O3(OH)3(C7H10O6)6]·20.5H2O (1) and (NH4)[Fe7(OH)6(C7H10O6)6]·(SO4)2·18H2O (2). Compounds 1 and 2 were characterized by analytical, spectroscopic (UV-vis, FT-IR, EPR, and Mössbauer) techniques, CV, TGA-DTG, and magnetic susceptibility measurements. The X-ray structures of 1 and 2 reveal heptanuclear assemblies of six Fe(III) ions bound by six doubly deprotonated quinates and one Fe(III) ion bound by oxido- and hydroxido-bridges (1), and hydroxido-bridges (2), all in an octahedral fashion. Mössbauer spectroscopy on 1 and 2 suggests the presence of Fe(III) ions in an all-oxygen environment. EPR measurements indicate that 1 and 2 retain their structure in solution, while magnetic measurements reveal an overall antiferromagnetic behavior with a ground state S = 3/2. The collective physicochemical properties of 1 and 2 suggest that the (a) nature of the ligand, (b) precursor form of iron, (c) pH, and (d) molecular stoichiometry are key factors influencing the chemical reactivity of the binary Fe(II,III)-hydroxycarboxylato systems, their aqueous speciation, and ultimately through variably emerging hydrogen bonding interactions, the assembly of multinuclear Fe(III)-hydroxycarboxylato clusters with distinct lattice architectures of specific dimensionality (2D-3D) and magnetic signature.

pH-specific synthetic chemistry and solution studies in the binary system of iron(III) with the alpha-hydroxycarboxylate substrate quinic acid: potential relevance to iron chemistry in plant fluids.

Iron is an essential metal ion in plant growth and development. Mobilization and further use of that metal by cellular structures in metabolic pathways entails the existence of soluble forms complexed with indigenous organic substrates, such as the low molecular mass d-(-)-quinic acid. In an effort to understand the relevant aqueous chemistry involving well-defined forms of iron, research efforts were carried out on the binary Fe(III)-quinic acid system. pH-specific reactions of FeCl(3).6H(2)O with quinic acid in a molar ratio 1:3 led to the isolation of the mononuclear Fe(III)-quinate complexes, K[Fe(C(7)H(11)O(6))(3)].(OH).3H(2)O (1), (NH(4))[Fe(C(7)H(11)O(6))(3)].(OH) (2), and Na[Fe(C(7)H(11)O(6))(3)].(OH).8H(2)O (3). Compounds 1-3 were characterized by analytical, spectroscopic techniques (UV/vis, FT-IR, Electron Paramagnetic Resonance (EPR), and Mossbauer spectroscopy), cyclic voltammetry, and magnetic susceptibility measurements. Compound 1 crystallizes in P2(1)3, with a = 15.1693(9) A, V = 3490.6(4) A(3), and Z = 4. Compound 2 crystallizes in P2(1)3, with a = 15.2831(9) A, V = 3569.7(4) A(3), and Z = 4. Compound 3 crystallizes in P2(1)3, with a = 15.6019(14) A, V = 3797.8(6) A(3), and Z = 4. The X-ray crystal structures of 1-3 reveal a mononuclear Fe(III) ion bound by three quinates in an octahedral fashion. Each quinate ligand binds Fe(III) through the alpha-hydroxycarboxylate group as a singly deprotonated moiety, retaining the alcoholic hydrogen. EPR measurements in solution suggest that 1 dissociates, releasing free quinate. Solution speciation studies of the binary system (a) unravel the aqueous species distribution as a function of pH and reagent molar ratio, and (b) corroborate the EPR results proposing the existence of a neutral Fe(III)-quinate complex form. The collective physicochemical properties of 1-3 formulate a well-defined profile for the Fe(III) assembly in aqueous media and project structural features consistent with solubilized Fe(III)-hydroxycarboxylate binary forms potentially mobilized into plant (bio)chemical processes.

SufA/IscA: reactivity studies of a class of scaffold proteins involved in [Fe-S] cluster assembly.

IscA/SufA proteins belong to complex protein machineries which are involved in iron-sulfur cluster biosynthesis. They are defined as scaffold proteins from which preassembled clusters are transferred to target apoproteins. The experiments described here demonstrate that the transfer reaction proceeds in two observable steps: a first fast one leading to a protein-protein complex between the cluster donor (SufA/IscA) and the acceptor (biotin synthase), and a slow one consisting of cluster transfer leading to the apoform of the scaffold protein and the holoform of the target protein. Mutation of cysteines in the acceptor protein specifically inhibits the second step of the reaction, showing that these cysteines are involved in the cluster transfer mechanism but not in complex formation. No cluster transfer from IscA to IscU, another scaffold of the isc operon, could be observed, whereas IscU was shown to be an efficient cluster source for cluster assembly in IscA. Implications of these results are discussed.

Catechol 1,2-dioxygenase from Pseudomonas putida in organic media--an electron paramagnetic resonance study.

The ability of an isolated isozyme of catechol 1,2-dioxygenase from Pseudomonas putida DSM 437 to function in a non-aqueous environment was investigated. The lyophilized enzyme is able to keep its catalytic function catalyzing the oxidation of catechol in n-hexane. Electron paramagnetic resonance (EPR) spectroscopy at liquid helium temperatures was applied to compare the properties of the non-heme iron of the enzyme in the organic solvent and in the aqueous solution. The catalytic performance of the enzyme in the organic solvent is correlated with the spectroscopic properties of the non-heme iron.

Mössbauer and EPR studies of the photoactivation of nitrile hydratase.

The alphabeta dimer of active nitrile hydratase from Rhodococcus sp. R312 contains one low-spin ferric ion that is coordinated by three Cys residues, two N-amide groups from the protein backbone, and one OH(-). The enzyme isolated from bacteria grown in the dark is inactive and contains the iron site as a six-coordinate diamagnetic Fe-nitrosyl complex, called NH(dark). The active state can be obtained from the dark state by photolysis of the Fe-NO bond at room temperature. Activation is accompanied by the conversion of NH(dark) to a low-spin ferric complex, NH(light), exhibiting an S = (1)/(2) EPR signal with g values of 2.27, 2.13, and 1.97. We have characterized both NH(dark) and NH(light) with Mössbauer spectroscopy. The z-axis of the 57Fe magnetic hyperfine tensor, A, of NH(light) was found to be rotated by approximately 45 degrees relative to the z-axis of the g tensor (g(z) = 1.97). Comparison of the A tensor of NH(light) with the A tensors of low-spin ferric hemes indicates a substantially larger degree of covalency for nitrile hydratase. We have also performed photolysis experiments between 2 and 20 K and characterized the photolyzed products by EPR and Mössbauer spectroscopy. Photolysis at 4.2 K in the Mössbauer spectrometer yielded a five-coordinate low-spin ferric species, NH(A), which converted back into NH(dark) when the sample was briefly warmed to 77 K. We also describe preliminary EPR photolysis studies that have yielded new intermediates.

Iron-sulfur center of biotin synthase and lipoate synthase.

Biotin synthase and lipoate synthase are homodimers that are required for the C-S bond formation at nonactivated carbon in the biosynthesis of biotin and lipoic acid, respectively. Aerobically isolated monomers were previously shown to contain a (2Fe-2S) cluster, however, after incubation with dithionite one (4Fe-4S) cluster per dimer was obtained, suggesting that two (2Fe-2S) clusters had combined at the interface of the subunits to form the (4Fe-4S) cluster. Here we report Mössbauer studies of (57)Fe-reconstituted biotin synthase showing that anaerobically prepared enzyme can accommodate two (4Fe-4S) clusters per dimer. The (4Fe-4S) cluster is quantitatively converted into a (2Fe-2S)(2+) cluster upon exposure to air. Reduction of the air-exposed enzyme with dithionite or photoreduced deazaflavin yields again (4Fe-4S) clusters. The (4Fe-4S) cluster is stable in both the 2+ and 1+ oxidation states. The Mössbauer and EPR parameters were DeltaE(q) = 1.13 mm/s and delta = 0.44 mm/s for the diamagnetic (4Fe-4S)(2+) and DeltaE(q) = 0.51 mm/s, delta = 0.85 mm/s, g(par) = 2.035, and g(perp) = 1.93 for the S = (1)/(2) state of (4Fe-4S)(1+). Considering that we find two (4Fe-4S) clusters per dimer, our studies argue against the early proposal that the enzyme contains one (4Fe-4S) cluster bridging the two subunits. Our study of lipoate synthase gave results similar to those obtained for BS: under strict anaerobiosis, lipoate synthase can accommodate a (4Fe-4S) cluster per subunit [DeltaE(q) = 1.20 mm/s and delta = 0.44 mm/s for the diamagnetic (4Fe-4S)(2+) and g(par) = 2.039 and g(perp) = 1.93 for the S = (1)/(2) state of (4Fe-4S)(1+)], which reacts with oxygen to generate a (2Fe-2S)(2+) center.

NO interacts with the tyrosine radical Y(D). of photosystem II to form an iminoxyl radical.

Incubation of photosystem II, PSII, membranes with NO for a few minutes results in the reversible elimination of the electron paramagnetic resonance (EPR) signal II from the oxidized Tyr Y(D)., presumably due to the formation of a weak Tyr Y(D).-NO complex [Petrouleas, V., & Diner, B. A. (1990) Biochim. Biophys. Acta 1015, 131-140]. Illumination of such a sample at ambient or cryogenic temperatures produces no new EPR signals. If, however, the incubation with NO is extended to the hours time range, illumination induces an EPR signal with resolved hyperfine structure in the g = 2 region. The signal shows the typical features of an immobilized iminoxyl radical (> C=NO.) with hyperfine values A(parallel) = 44 G, A(perpendicular) = 22 G, and A(iso) = 29.3 G. The following observations suggest that the iminoxyl signal is associated with PSII: (a) the signal results from an immobilized species at room temperature probably associated with a membrane-bound component, (b) the abundance of the signal is (sub)stoichiometric to PSII, (c) the signal is light-induced, (d) some of the treatments that affect PSII (Tris, Ca2+ depletion, high-salt wash) severely diminish the size of the signal, and (e) the development of the signal correlates with the release of Mn. In addition, the following observations suggest that the iminoxyl signal results from an interaction of Y(D). with NO: (a) the evolution of the signal correlates with the loss in reversibility of the Tyr Y(D).-NO interaction and (b) the size of the signal correlates with the initial amount of oxidized Tyr Y(D). It is accordingly proposed that during the incubation with NO, a weak Tyr Y(D).-NO complex is rapidly formed and is then slowly converted to a tyrosine-nitroso adduct. Light-induced oxidation of the latter produces the iminoxyl radical. The nitrosotyrosine is expected to have an oxidation potential significantly lower than the parent tyrosine and can act as an efficient electron donor in PSII even at cryogenic temperatures. It is probably this lowered redox potential of the tyrosine Y(D) that explains the release of Mn concomitant with the formation of the nitroso species.

Effects of photoinhibition on the QA-Fe2+ complex of photosystem II studied by EPR and Mössbauer spectroscopy.

Effects of photoinhibition on the iron-quinone electron acceptor complex of oxygen-evolving photosystem II have been studied using low-temperature EPR and Mössbauer spectroscopy. Photoinhibition of spinach photosystem II membrane particles at 4 degrees C decreases the EPR signal arising from the interaction of QA- with Fe2+ to 30% in 90 min under our conditions. The free radical EPR signal from QA- induced by cyanide treatment of the iron [Sanakis, Y., et al. (1994) Biochemistry 33, 9922-9928] declines with the same kinetics as the QA-Fe2+ EPR signal. In contrast, Fe2+ is present in about 70% of the centers after 90 min of photoinhibition, as shown by its EPR-detected interaction with NO and by its Mössbauer absorption. Complete oxidation of this Fe2+ population to Fe3+ by ferricyanide is possible only in the presence of glycolate, which lowers the redox potential of the Fe3+/Fe2+ couple. In a fraction of PSII centers, which reach 30% after 90 min of photoinhibition, the iron cannot be detected. It is concluded that photoinhibition of oxygen-evolving photosystem II affects both QA and Fe2+. However, the photoinhibitory impairment of the QA redox functioning precedes the modification of the non-heme iron. In a considerable portion of the photoinhibited centers, which do not have functional QA, the non-heme iron is still present and redox active, but its redox potential is increased relative to that in the normal centers. This is probably due to a minor modification of the bicarbonate ligation site.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyanide binding at the non-heme Fe2+ of the iron-quinone complex of photosystem II: at high concentrations, cyanide converts the Fe2+ from high (S = 2) to low (S = 0) spin.

The primary electron acceptor complex of photosystem II, QAFe2+, can bind a number of small molecules at the iron site, including cyanide [Koulougliotis, D., Kostopoulos, T., Petrouleas, V., & Diner, B. A. (1993) Biochim. Biophys. Acta 1141, 275-282)]. In the presence of NaCN (30-300 mM) at pH 6.5, the reduced state, QA-Fe2+, produced either by illumination at < or = 200 K or by reduction in the dark with sodium dithionite, is characterized by a g = 1.98 EPR signal. The light- or dithionite-induced g = 1.98 signal decays with increasing pH above 6.5 and is almost totally absent at pH 8.1 and NaCN concentrations above 300 mM. However, at high pH (8.1), the g = 1.98 signal still forms transiently before it decays with a t1/2 of approximately 30 min in spinach BBY preparations treated with 100 mM NaCN. Complementary to the disappearance of the g = 1.98 signal with increasing pH or incubation time, a new EPR signal develops at g = 2.0045. This signal has the characteristics of the semiquinone, QA-, uncoupled from its magnetic interaction with the iron. Prolonged incubation of a high pH, high cyanide treated sample in a cyanide-free medium at pH 6 restores the ability of the sample to develop the cyanide-induced g = 1.98 signal at pH 6.5. This indicates that the iron is not physically dissociated during the high pH cyanide treatment. The high pH, high cyanide effects are accompanied by the conversion of the characteristic Fe2+ (S = 2) Mössbauer doublet [isomer shift (Fe) = 1.19 mm/s, quadrupole splitting = 2.95 mm/s] to a new one with parameters (isomer shift = 0.26 mm/s, quadrupole splitting = 0.36 mm/s) characteristic of an Fe2+(S = 0) state.(ABSTRACT TRUNCATED AT 250 WORDS)